Handheld electronic devices and other portable electronic devices are becoming increasingly popular. Examples of handheld devices include handheld computers, cellular telephones, media players, and hybrid devices that include the functionality of multiple devices of this type. Popular portable electronic devices that are somewhat larger than traditional handheld electronic devices include laptop computers and tablet computers.
Handheld wireless communications devices often have several functions that involve digital audio signal processing. For example, consider their use as a mobile telephony device (e.g., a cellular telephone handset). Following a call set up or connection phase, a simultaneous two-way voice conversation between a local user of the device and another (remote) user in a telephone call may be enabled as follows.
A so-called uplink chain in the device is responsible for digitizing the local user's speech that has been detected by a built-in microphone. This may result in a raw digital audio signal or bit stream, e.g. a pulse code modulated, PCM, audio signal or bitstream. The uplink chain then digitally codes the raw signal, to remove its redundant content. For instance, a 64 kbits/sec raw speech bitstream may be encoded as a 14 kbits/sec bitstream, without causing a noticeable drop in sound quality. Next, the uplink chain modulates a RF carrier signal with the coded signal (and other information regarding the call). An antenna of the device is then driven with the modulated RF carrier. The local user's speech is thus transmitted to the cellular telephone network.
To enable the above-mentioned two-way conversation, a downlink chain is provided in the device, so that the local user can hear the remote user's speech. The downlink chain operates in parallel with or simultaneously as the uplink chain, to enable the real-time two-way conversation. The downlink chain may essentially perform the reverse of the uplink chain's operations. Thus, an antenna of the device outputs a downlink RF signal sent by the cellular telephone network. The downlink chain then demodulates the downlink RF signal to yield a so-called baseband signal. The latter contains a coded audio signal, which includes an encoded version of the captured speech of the remote user. The coded audio signal is decoded (e.g., into a PCM bitstream), converted to analog format and then played to the local user, through a receiver or speaker of the device. To render higher quality or better sound when an audio signal is played back, various signal processing operations may be performed on the digital audio signal in both the downlink and uplink chains. These may include noise filtering or noise suppression (sometimes referred to as noise cancellation), gain control, dynamic range compression (or simply compression), energy limiting, and echo cancellation.
Compression results in loud sounds over a certain threshold being reduced in level, while quiet sounds remain untreated. In this way, it reduces the dynamic range of an audio signal. This may be done to improve audibility in noisy environments. For example, a comfortable listening level for quiet sounds in a noisy environment makes the loud sounds too loud. Compression reduces the level of the loud sounds (by reducing the gain for those sounds), but not the quiet sounds, so that volume can be raised to a point where the quiet sounds are more audible without the loud sounds being too loud. A compressor decreases gain (level) of an audio signal if its amplitude exceeds a certain threshold. Typically, an input/output ratio is defined that determine the amount of compression. For example, a 4:1 ratio means that an input signal overshooting a threshold by 4 dB will leave the compressor 1 dB above the threshold. Also, in practice, a compressor exhibits a delay before its output level is actually reduced to the required level—this is referred to as the attack phase.
Somewhat similar to a compressor, a hard limiter also limits loud sounds. However it does so in a much more abrupt manner, in effect exhibiting a much higher ratio and a much shorter attack phase. Hard limiting is typically used as a safety device than as a sound-sculpting tool.